To avoid tolls past Rouen, but at cost of time, distance and more diesel:
Rouen, to Caen either A13 peage or Bernay and N13. Caen to Rennes free autoroute. Rennes via Nantes and Cholet to Poitiers, free all the way down. (N10 South of Poitiers is a nasty road nowadays: all dual 80kph, no overtaking, and lots of police to catch those who tailgate and drive like loons) but it does avoid the peage and still.afew decent restaurants on it.
Have you priced Le Havre and Caen boats? Not cheap but less peage and under 9 hrs from boat to Bx. Depends on your loading point UK and timings.